(1) To restrict a variable to discrete values, each of which is normally an integral multiple of the same quantity. (2) To process a range of grey shades, from maximum to minimum, such that the entire range is divided into contiguous intervals of normally equal lengths, each being assigned an integer value unique to the grey shade corresponding to it. Output voltages from the ETM+ and TM sensors aboard Landsat 4, 5 and 7 are quantized into 256 discrete values prior to transmission to the ground. A total of 256 brightness values are therefore possible in each band of ETM+ and TM imagery.